1. Field of the Invention
This invention relates to a sewing machine provided with a needle bar rocking mechanism for rocking a needle bar leftward and rightward and a thread tension releasing mechanism for releasing a needle thread from tension applied thereto by a thread tension applying mechanism.
2. Description of the Related Art
Household sewing machines comprise a sewing arm including a head provided with a needle bar rocking mechanism, a thread tension applying mechanism and a thread tension releasing mechanism for releasing a needle thread from tension applied thereto by the thread tension applying mechanism. The needle bar rocking mechanism rocks a needle bar in a direction crossing the direction (e.g., forward and backward) in which a workpiece cloth is fed, for example, leftward and rightward. The needle bar rocking mechanism comprises an actuator composed of a stepping motor etc. The needle bar is rocked leftward and rightward by the actuator together with a needle bar bracket mounted on a shaft directed forward and backward, so that various types of stitches such as a zigzag stitch are formed on the workpiece cloth.
The thread tension applying mechanism applies tension to a needle thread supplied from a bobbin serving as a thread supply to be caused to pass through the eye of a sewing needle mounted on a lower end of the needle bar. The thread tension applying mechanism comprises a pair of oppositely disposed thread tension discs or fixed and movable thread tension discs, and a spring for urging the movable thread tension disc toward the fixed thread tension disc. The needle thread supplied from the bobbin via a needle thread take-up lever to the sewing needle is en route held between the fixed disc and the movable disc urged by the spring to be pressed against the fixed disc, so that tension is applied to the needle thread.
The thread tension releasing mechanism comprises an actuator, such as a solenoid actuator, for moving the movable thread tension disc away from the fixed thread tension disc. The movable disc is parted from the fixed disc so that the needle thread is released from the tension.
In the above-described conventional sewing machine, the needle bar rocking mechanism and the thread tension releasing mechanism require individual actuators respectively. This increases the manufacturing cost of the sewing machine and the size of the sewing machine head provided with the needle bar rocking mechanism, the thread tension applying mechanism and the thread tension releasing mechanism.